Adjusting an animation
You really should open up the Graph Editor...

I you select the arms, you should be able to uncheck all animations at first by clicking the upmost EYE icon



Then select one (open the eye) of the rotation channels (you may have X Y Z Euler Rotation unlike in the image). Then keep your mouse cursor over the curve and press A to select all keys, then press G to grab them and then Y to restrict the Y axis and then just move your mouse up or down to adjust the rotation of the arm.



Hope this helps